Plasma polymerization is a technique used for depositing polymer-like organic materials, usually in the form of thin films, onto surfaces in contact with or near a plasma discharge. Unlike conventional polymers, plasma polymers do not consist of long chains of monomeric repeat units with sparse "cross-links" connecting the chains. Instead, they are highly branched, three-dimensional interlinked monomer-derived networks which result from fragmentation and dissociation in the plasma in which the film-forming reactant species are generated. Plasma polymerized films are formed from organic monomers and are in general, pinhole-free, dense and amorphous. When compared with conventional polymer films made from the same monomer(s), plasma polymers exhibit better adhesion, and improved chemical and mechanical resistance. Furthermore, the properties of the deposited films can be changed by varying the deposition parameters.
Plasma polymerized films are formed in an apparatus that typically consists of three parts: (1) a vacuum system, (2) an electrical excitation system for generating a plasma, and (3) a monomer gas delivery system. As monomer molecules flow through the vacuum chamber, the plasma discharge energizes and disassociates the monomer molecules into neutral particles and reactant fragments in the form of electrons, ions and free radicals. As these reactant fragments recombine on a substrate, a highly branched and cross-linked three-dimensional network is formed.
Industrial applications of plasma polymers include surface modifications, barrier coatings, and dielectric-, photoresist, and waveguiding films for microelectronics and photonics.
The macroscopic properties of plasma polymers are typically very different from those of conventional linear polymers made from the same starting monomer(s). Indices of refraction and mechanical resistance have been the most often characterized macroscopic physical properties of plasma polymers, and they are much different from those of conventional cross-linked linear polymer films, even when made from the same monomer.
Although macroscopic properties are manifestations of microscopic properties, many microscopic physical and chemical properties of plasma polymers have not yet been well characterized. In fact, because of the heterogeneous character of the linkages in the plasma polymer, it is difficult to do such characterizations. One microscopic property which is manifested in macroscopic differences between conventional polymers and plasma polymers is color. The color of many plasma polymers derived from single monomers is yellow, even though the corresponding conventional polymers produced from the same monomer are clear under white-light illumination. The yellow color of plasma polymers such as methyl methacrylate, ethylene, and hexamethyldisiloxane indicates absorption of light in the blue, violet and ultraviolet region of the visible spectrum. Blue-violet absorption is confirmed by ultraviolet-visible-near infrared transmission analysis which reveals appreciable absorption at wavelengths less than 800 nm and markedly increased absorption below 450 nm. Absorption of blue and violet light has heretofore been thought solely responsible for the yellow tint of plasma polymers.